[whatwg] proposal: Add support for masking to canvas

[Rik Cabanier]

luminance would be calculated the same as for compositing and blending:

https://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html#blendingnonseparable

This is done with no adjustment for gamma.

Maybe we can add real ICC color management later so you can blend/mask in
other RGB colorspaces or Lab.

Rik

On Tue, Jan 29, 2013 at 5:34 PM, Jatinder Mann <jmann at microsoft.com> wrote:

>  Seeing that alpha and luminance masking is definitely useful, I would
> like to see them added to the spec.****
>
> ** **
>
> We may want to ensure that the spec provides a very clear mathematical
> definition of luma. Luma, luminance, and luminosity are technically
> different things. One specific feedback is whether the spec will be
> specifying if user agents should assume linear gamma space or 2.2 gamma
> space. Basically all images are natively stored in 2.2. gamma space, but
> luminance is typically calculated assuming linear gamma. I believe 2.2
> gamma would be faster, as there is no need for the conversion. ****
>
> ** **
>
> Thanks,****
>
> Jatinder****
>
> ** **
>
> *From:* Rik Cabanier [mailto:cabanier at gmail.com]
> *Sent:* Thursday, January 24, 2013 1:42 PM
> *To:* public-canvas-api at w3.org; whatwg at whatwg.org
> *Subject:* proposal: Add support for masking to canvas****
>
> ** **
>
> All, ****
>
> ** **
>
> the canvas API does not have a convenient way of supporting masking today.
> ****
>
> There is some support with the compositing operators but it requires quite
> a bit of work on the author's part as well as a temporary canvas. In
> addition, there is no way to implement a luminosity mask without reading
> the pixels and doing the math yourself.****
>
> ** **
>
> I was thinking of proposing the following new API calls for the 2d canvas
> context:****
>
> enum CanvasMaskType { luminosity, alpha};
> void maskImage(image, sx, sy, sw, sh, dx, dy, dw, dh, maskType, invert,
> clipToMask);
> void resetMask();****
>
>  ** **
>
> 'maskImage' will set the mask in the current graphics state which makes it
> subject to save/restore. parameters:****
>
> image = HTMLImageElement or HTMLCanvasElement or HTMLVideoElement. ****
>
>  Points to set of pixels that will be used as the mask source ****
>
>  ** **
>
> sx, sy, etc = describes what portions of the source and destination canvas
> to target. (This is similar to drawImage [1])****
>
> ** **
>
> maskType = sets what type of mask.****
>
> luminosity = (default) use the luma value of the pixel****
>
>   alpha = just look at the alpha channel. ignore the color channel ****
>
> invert = boolean. (default = false) invert the mask values ****
>
> ** **
>
> clipToMask = boolean (default = true) the area outside of the mask pixels
> clipped away. If false, that area is not affected by the mask.****
>
> ** **
>
> 'maskImage' will set the mask in the current graphics state which makes it
> subject to save/restore.****
>
>  ** **
>
>  'resetMask' will remove the mask from the current graphics state.****
>
> ** **
>
> Some people told me that maskImage is taking too many parameters and that
> a dictionary would be better.****
>
> ** **
>
> What do people think? Is this useful enough to add to the API?****
>
> ** **
>
> Rik****
>
> ** **
>
> 1:
> http://www.w3.org/html/wg/drafts/2dcontext/html5_canvas/#dom-context-2d-drawimage
> ****
>